Rotary single-phase electromagnetic servo actuator comprising an actuator and a position sensor

ABSTRACT

A single phase electromagnetic servo-actuator that includes a rotary actuator which moves a mobile member along a limited travel including a 2N ferromagnetic pole stator structure and at least one excitation coil, the stator structure being made of a material with high magnetic permeability and a rotor having ferromagnetic yoke and a thin magnetized portion of 2N pairs of axially magnetized poles, in alternate directions, and a rotor angular position sensor, the thin magnetized portion being a separate element from the ferromagnetic yoke.

This is a Continuation Application of U.S. application Ser. No.11/917,611, filed Mar. 12, 2008, the entire content of which isincorporated herein by reference. U.S. application Ser. No. 11/917,611is the National Stage of PCT Application No. PCT/FRO6/01359, filed Jun.15, 2006, and claims the benefit of priority under 35 U.S.C.§119 fromFrench Patent Application No. 05 06065, filed Jun. 15, 2005.

The present invention relates to an electromagnetic servo-actuatorcomprising a rotary single-phase actuator producing a constant torque ona limited angular travel, including a rotor consisting of aferromagnetic material yoke and 2N pairs of axially magnetized poles, inalternate directions and a stator structure respectively including 2Nferromagnetic poles and at least one coil, and a rotor position sensor.

This combination of an actuator with a sensor is intended to move, inrotation and in a non-limitative way, an air intake valve for aninternal combustion engine.

It is already known, in the prior art, to combine an actuator and anelectromagnetic sensor for actuating a valve and more particularly formotor vehicles. The solutions used generally lead to solutions havingvarious disadvantages: significant overall dimensions because of theintrinsic overall dimensions of both actuator and sensor members,disturbance of the signal from the sensor during the operation of theactuator, difficult indexing between the sensor and the valve position.

One of the objects of the present invention consists in remedying thesedisadvantages by providing an innovative solution for the man skilled inthe art.

For this purpose, an object of the present invention is to provide anelectromagnetic servo-actuator comprising an innovative actuator+sensorassembly, wherein that the sensor is incorporated on the actuator yokein a simple way, without any influence from the magnetic field createdby the actuator on the sensor.

Advantageously, the yoke of the actuator is made of a ferromagneticmaterial with a high magnetic permeability which creates a magneticarmoring and avoids any influence from the engine on the sensor signal.In addition and preferably, the magnetism-sensitive probe, the receiverof the magnetic field, is placed in at least one neutral plane relativeto the stator poles of the actuator where the influence of the enginemagnetic field is null.

In a preferred embodiment, the sensor used is of the type described inpatent FR2670286. This type of sensor makes it possible to work around a0 Gauss signal, insensitive to the variations of temperature. Itincludes a magnetic field emitter in the form of a ring magnetpositioned between two collection circuits formed by two ferromagneticarcs which partially enclose the ring magnet. The adjustment of such 0Gauss position requires an indexing of the sensor magnet relative to thesensor stator. However, it is possible to use other types of sensors andone of the objects of this application is to allow the utilization,according to various embodiments, of various types of sensors usingmagnetism-sensitive probes or inductive sensors.

Another object of the invention is also to allow the utilization of adigital type sensor, which is used only for detecting the beginningand/or the end of the travel.

In the present state of the art are known electromagnetic actuatorscapable of performing the motion of a valve with a proportional constanttorque. The publication FR2670629 discloses, for this purpose, anactuator with a rotor having 2N pairs of magnetized poles producing atorque proportional to the feed current and allowing the self-centeringof the rotor, thanks to the combination of an axial stop and the axialelectromagnetic force of the magnets on the stator part.

However, the embodiment of this actuator does not allow an optimumutilization and an industrial production. Besides, the significantoverall dimensions of the combination of such an actuator with anangular position sensor, (a combination which is often indispensable inmotor vehicle applications, for safety reasons), is redhibitory for arealistic utilization of the assembly.

Another object of the present invention is thus to provide an actuatorhaving an innovative structure making it possible to obtain a strongconstant torque, proportional to the control current, to limit thenumber of parts, while improving the thermal behavior of the actuatorduring its operation and to make the servo-actuator applicable in theindustry.

Preferably, the stator poles are produced using a so-called “coldheading method” in a cast material, which provides a high ferromagneticquality. The stator poles thus produced are then mechanically expelledinto a base plate made of a ferromagnetic material which provides anoptimum magnetic joint and limits the occurrence of a residual torqueduring the displacement of the rotor, comprising a yoke+the motormagnetized poles, above the stator poles. The arrangement of themagnetic poles on the base defines the seat of a stop support, intendedto receive a stop on which the rotor comes to rest, thanks to a shouldermade on each of the 4 poles during the production thereof, while usingthe cold heading method.

According to an advantageous embodiment, the stator assembly thus formedcan then be overmolded with a thermoplastic polymer of the LCP type(Liquid Crystal Polymer). Using such methods makes it possible toproduce the guiding bearing and the support of the rotor stop in onlyone operation and in a simple and cost effective way, while providing animprovement of the temperature resistance, through the easier dischargeof the electric power dissipated by Joule effect, by the electric supplycoils.

Another object of the present invention is to provide a servo-actuatorprovided with a locking and unlocking system capable of blocking anyexternal mechanical action applied to the actuator-driven outlet pin,without hindering the actuator-driven motion of the member to be movedin both directions of rotation of this member. Such combination of theservo-actuator+the locking and unlocking system makes it possible toproduce an undersized actuator for a given application since externalreaction forces which are exerted on the controlled rotation pin are nolonger to be overcome. This makes it possible to spare room and to havea decisive cost saving for an important number of applications and makesit possible to use such actuator in multiple applications which couldnot be used without such combination.

Understanding the description will be easier while referring to theappended drawings, in which:

FIG. 1 shows a three-quarter view of the servo-actuator described in thepresent invention, in its preferred embodiment;

FIGS. 2A and 2B show separate views of the servo-actuator, according toa first embodiment of the sensor, without the overmolding assembly;

FIG. 3 shows a separate view of the servo-actuator according to a secondembodiment of the sensor, without the overmolding assembly;

FIG. 4 shows a separate view of the servo-actuator, according to a thirdembodiment of the sensor, without the overmolding assembly;

FIG. 5 shows a separate view of the servo-actuator, according to afourth embodiment of the sensor, without the overmolding assembly;

FIG. 6 shows a separate view of the lid of the servo-actuator intendedto be fixed on the overmolded stator assembly;

FIG. 7 shows the stator assembly with its LCP overmolding;

FIG. 8 shows the stator assembly without the LCP overmolding;

FIG. 9 shows an exploded view of the stator assembly with its LCPovermolding, the rotor and its axial stop;

FIG. 10 shows a view of the horizontal overmolded stator assembly;

FIG. 11 shows an isolated view of a ball stop in a preferred embodiment,which can be used to provide the positioning and the free rotation ofthe actuator rotor on the stator part;

FIG. 12 shows a separate view of the servo-actuator without theovermolding according to a second embodiment of the actuator;

FIG. 13 shows a separate view of the servo-actuator of FIG. 1 associatedwith a mechanical locking and unlocking system;

FIG. 14 shows an alternative servo-actuator of FIG. 12 for which therotor has a through pin;

FIG. 15 shows an alternative servo-actuator of FIG. 12 using a sensorcapable of letting the actuator rotor pin through, upwards.

FIG. 16 shows an alternative servo-actuator according to a fifthembodiment of the sensor.

FIG. 1 shows the servo-actuator 1, such as shown in a preferredembodiment. It is composed of an overmolded stator assembly 2 on whichone or several studs 3 can be placed to fasten the servo-actuator 1 onan external frame, as well as a lid 4 integrating an assembly ofelectric connections 5 used for driving the actuator and electromagneticsensor, and for producing the electric signal from the sensor, forexample including a Hall probe. The lid 4 is integrally linked to theovermolded stator assembly 2 by clipping cutout legs 6 extending the lid4 with protruding grippers 7 of said overmolded stator assembly 2. Theface opposite the lid 4 has a bearing 8 formed in the overmolded statorassembly 2 through which the actuator pin (not shown) extends to befixed on an outside members to be positioned and moved in rotation.

In FIGS. 2A and 2B, is shown the servo-actuator 1, consisting of anactuator 9 and a sensor 21, without the overmolding elements nor the lid4, and according to a first embodiment. The actuator 9 which composes itis composed of a first stator assembly 10 comprising here 4 stator poles11, extending in the axial direction of the actuator, fixed on the base12 and a rotor 13 including an axially alternately magnetized disc 14having four pairs of magnetic poles and one yoke 15 made of aferromagnetic material. Such yoke 15 is axially extended to support asecond magnet 16, having a ring shape, including two pairs of magnetizedpoles in the radial direction, alternatively. This second magnet 16 isthus integral with the yoke 15 and rotates inside a second statorassembly 17 which is stationary with respect to the stator. It isconstituted of two ferromagnetic arcs 18 partially enclosing the secondring magnet 16 between which a magnetism-sensitive probe 19 having aHall effect is positioned. Preferably, the magnetism-sensitive probe 19is positioned in such a way that preferably the magnetism-sensitiveprobe 19 is placed such that its detection sensitive element is placedin the middle plane 20 of at least one of the stator poles 11, an areafree of any magnetic induction. The ferromagnetic arcs 18 are thinner atthe level of the magnetism-sensitive probe 19, which makes it possibleto concentrate the flow magnetic flow from the second magnet field 16.The flow collected at the magnetism-sensitive probe 19 is thus a linearfunction of the position of the yoke 15 and thus of the external memberdriven by said yoke 15 with the help of a through pin (43).

FIG. 3 shows the servo-actuator 1 according to a second embodiment ofthe sensor 21 and without any overmolding or lid 4. The yoke 15 of theactuator has a U ferromagnetic shape 22 having a coercitive field forcelower than has 500 Oersteds which has a residual axial magnetization. Inthe axis of such U ferromagnetic shape 22 corresponding to theintersection of the middle planes 20 of symmetry of the stator poles 11,is placed a Hall probe 24, for instance of the Melexis 90316 type whichhas at least 2 Hall sensitive elements. Such sensitive elements sensethe induction generated by the U ferromagnetic shape 22 according to twoperpendicular directions. Preferably, but not limitatively, these twoorthogonal directions are defined by the middle planes 20 of symmetry ofthe stator poles 11. The output signal obtained is a linear function ofthe rotation angle of the U ferromagnetic shape 22 and the signal is notdisturbed by the operation of the actuator 9, since the sensitiveelements of the Hall probe 24 are placed close to the axis of suchactuator 9, an area free of any magnetic induction.

FIG. 4 shows the servo-actuator 1 according to a third embodiment of thesensor 21 and without any overmolding or lid 4. The yoke 15 of theactuator has a diametrically magnetized cylindrical ferrite magnet 25.In the axis of this cylindrical ferrite magnet 25, corresponding to theintersection of the middle planes of symmetry of the stator poles 11, isplaced a Hall effect probe 24, for instance of the Melexis 90316 type,which has further at least two Hall sensitive elements. Such sensitiveelements sense the induction generated by the cylindrical ferrite magnet25 according to two orthogonal directions. Preferably but notlimitatively, such two orthogonal directions are defined by the middleplanes 20 of symmetry of the stator poles 11. The output signal obtainedis a linear function of the rotation angle of the cylindrical ferritemagnet 25 and the signal is not disturbed by the operation of theactuator 9 since the sensitive elements of the hall probe 24 are placedclose to the pin of such actuator 9, an area free of any magneticinduction.

FIG. 5 shows the servo-actuator 1 according to a fourth embodiment ofthe sensor 21 and without any overmolding or lid 4. The yoke 15 of theactuator has a U shape 26 and is made of any material which provides thepositioning of the magnet 27 in the slot 28 of the U shape 26. In theaxis of the U shape 26, corresponding to the intersection of the middleplanes of symmetry of the stator poles 11, is placed a Hall probe 24,for example of the Melexis 90316 type, which has at least 2 Hallsensitive elements according to two orthogonal directions. Preferably,but not limitatively, such two orthogonal directions are defined by themiddle planes 20 of symmetry of the stator poles 11. Such sensitiveelements sense the induction generated by the magnet 27. The outputsignal obtained is a linear function of the rotation angle of the magnet27 and the signal is not disturbed by the operation of the actuator 9,since the sensitive elements of the Hall probe 24 are placed close tothe axis of such actuator 9, an area free of any magnetic induction.

FIG. 6 shows the lid 4 closing the servo-actuator 1 according to a firstembodiment of the sensor 21. It contains the stator assembly of thesensor 21, as well as the magnetism-sensitive probe 19 used to measurethe magnetic flux produced by the sensor 21 described in FIGS. 2A and2B. In this embodiment, the magnetism-sensitive probe 19 is a Hall probewhich receives, proportionally to the rotor position 13, the magneticflux produced by the second sensor magnet 16 positioned on the yoke ofthe rotor 13 of the actuator 9. The position of the stator assembly ofthe sensor 21 and of the magnetism-sensitive probe 19 relative to thestator poles 11 is provided by the overmolding of the servo-actuator 1.In combination with the second sensor magnet 16, which is alsopositioned on the yoke 15 relative to the axially magnetized disc 14 ofthe actuator 9, the assembly thus formed makes it possible to obtain ina repetitive way in production a servo-actuator 1, which is compact andknows the absolute position of the rotor 13. The lid 4 obtained hascutout legs 6 which are intended to provide the connection and the holdof the lid 4 with the overmolded stator assembly 2. It also has twoelectric connection elements 29 intended to provide the electricconnection of the overmolded stator assembly 2. Such electric connectionelements 29 which are in the form of slotted rotor plate, which is ashape providing an elastic connection insensitive to positioningtolerances between the stator assembly 2 and the lid 4 during theirassembly. The electric connection of the actuator 9 and the sensor 21 iscarried out at the level of the electric connection assembly 5 of thelid 4. Using an external connector (not shown) it is thus possible toconnect the servo-actuator 1 of the present invention.

FIG. 7 shows the overmolded stator assembly 2. In an advantageousembodiment, the overmolding of the stator assembly 10 is made in athermoplastic material such as a LCP (Liquid Crystal Polymer). Suchovermolding, encompassing the stator poles 11 and the electric supplycoils 33, makes it possible, on the one hand, to more easily evacuatethe heat produced by a Joule effect at the level of the coils 33 duringthe supply thereof, and on the other hand makes it possible to produce,without any additional step, a stop support 31 made of LCP, coming torest on the shoulders 48 of the stator poles 11, as well as a bearing,not shown in the Figure, on the face opposite the lid 4 of the actuator9. In addition, the overmolding makes it possible to provide themechanical connection of the overmolded stator assembly 2 with the lid 4thanks to the presence of protruding grippers 7, on which the cutoutlegs of 6 of the lid 4 are clipped and thanks to the presence of agroove 30 in the overmolding which is intended to receive an adhesivepaste ensuring the contact of the overmolded stator assembly 2 with thelid 4. The electric connection is carried out at the level of thecross-shaped electric connection area 33, which allows the elasticconnection of two electric connection elements 29 of the lid 4 in theform of slotted rotor plates. Eventually, the overmolding has ananti-vibration function which protects the servo-actuator 1 against theexternal vibration during the operation of the assembly in theapplication.

FIG. 8 shows a separate view of the stator poles 11 of the actuator,mechanically expelled into a base 12 made of a ferromagnetic material.The four stator poles 11 have, each, a pole shoe 32 at the level oftheir heads, which makes it possible to increase the useful angulartravel and to reach a value close to 80°. The association of the fourstator poles 11 and the pole shoes 32 with the magnetized poles of therotor makes it possible to obtain a torque ratio on an optimum number ofpoles. In an advantageous embodiment, the stator poles 11 are obtainedby means of the so-called “cold heading” method making it possible toobtain them in only one operation. To provide good magnetic fluxconduction, the stator poles 11 are mechanically expelled into a base 12made of a ferromagnetic material. The flatness of the stator poles 11 ason at the level of their heads is provided from the rear of the base 12,where the base of the stator poles 11 is not flat. Thanks to themechanical expelling action, the magnetic joint obtained is optimum andmakes it possible not to create any magneto-static residual torque, andto provide the proper conduction of the magnetic induction flux betweenthe stator poles 11 without deforming the currentless torque of theactuator 9. Advantageously, such stator poles 11, has shoulders 48 whichare used as seats for a stop support 31 of the rotor 13 of the actuator9. The electric supply coils 33 used for generating the magnetic fluxused by the actuator 9 are placed on each of the stator poles 11. InFIG. 8, only one of such coils 33 is shown.

FIG. 9 shows an exploded view of the overmolded stator assembly 2, anaxial stop 34 and the rotor 13 of the electromagnetic actuator 9provided for the present invention. The axial stop 34 comes to rest onthe stop support 31 constituted of the overmolded stator assembly 2. Therotor 13 of the actuator, constituted of a yoke 15 made of aferromagnetic material and an axially magnetized disc having four pairsof magnetized poles in alternate directions, then comes in contact withthe axial stop 34, which is guided thanks to its pin 43 which isintegral with the bearing 8 made in the overmolded stator assembly 2 onthe face opposite the lid 4. Thanks to this advantageous embodiment, themagnetic attraction of the rotor 13 on the stator poles 11 associatedwith the axial and radial guiding constituted by the axial stop 34 andthe bearing 8, respectively made in the overmolded stator assembly 2,provides at least self-centering and the parallelism of the rotor 13with respect to the surface of the stator poles 11. At the back of theovermolded stator assembly 2 can be placed one or several studs 3 whichare threaded and capable of providing the fastening of theservo-actuator on an external element for example the air intake valveof a vehicle. The shape and the number of the studs 3 are not limitedand for example two studs 3 can be positioned as indicated in FIG. 10.Such stud 3 can be associated with a guiding element 35 for positioningthe servo-actuator 1 prior to fixing it. Such guiding 35 and fasteningelements are generally screwed into the base 12 or the stator pole base11, since the back of the actuator 9 is free of any over molding inorder to allow a better evacuation of heat emitted in Joule by theactuator 9 during the operation in the application. The positioning ofthe actuator is further facilitated flat lock 36 at the base level 12.

FIG. 11 shows a ball thrust bearing 34 which can be used in anon-limitative way in the present invention. Preferably, such ballthrust bearing 34 is composed of 3 simple assemblies: an assembly of twoflanges 37 containing an assembly of balls 38 sliding between the twoflanges 37 which are themselves retained in a ball bearing housing 39generally made of plastic material and providing a cohesion of the ballthrust bearing 34. Both flanges 37 can thus slide with respect to eachother, independently. Such ball thrust bearing 34 is placed on theovermolded stator assembly 2 and it supports the rotor 13, provides itsself-centering and enables it to rotate when it is placed on theovermolded stator assembly 2, while limiting the friction to a minimum.

FIG. 12 shows a servo-actuator 2 according to the teachings of thepresent invention and according to a second embodiment of the actuator 9and for which the details of the embodiment of the actuator 9 disclosedin the present application can be applied. Such actuator 9 thus shows arotor 13 constituted of a yoke 15 on which a magnet 40 having two pairsof pole is made integral. Such rotor is positioned on ball thrustbearing 34 which is itself positioned on a stator part 41 composed oftwo ferromagnetic poles 49 and an electric supply coil 42. In thisexample which is non-limitative, as regards the type of the sensor used,the pin 43 is U-shaped in its upper part and made of a material having acoercitive field force lower than 500 Oersteds and having a residualaxial magnetization as indicated in FIG. 3 in the second embodiment ofthe sensor 21.

In FIG. 13, the servo-actuator 1 disclosed in the present application isassociated with the locking and unlocking system 44, here a mechanicalsystem the function of which consists in blocking the externalmechanical actions exerted on the output pin. Therefore, it has amechanical locking system 45 located under the rotor 13 of the actuator9 and composed of two distinct parts moving with respect to one anotherand the function of which is to enable the locking of any externalaction without preventing the actuator-driven motion. The addition ofsuch a locking/unlocking system 44 makes it possible to undersize theactuator for a given overall dimension, since it is no longer sensitiveto the fluctuations of the external load exerted on the member driven bythe pin 43 connected in a stationary way to the rotor 13. Such system isthen constituted of a mechanical assembly, but such embodiment is notlimitative and can be made with an assembly of another nature(electromagnetic, hydraulic, and pneumatic).

FIG. 14 shows an alternative to the servo-actuator 1 of FIG. 12 forwhich the pin 43 of the rotor 13 goes through its support towards theback of the actuator 9, with a view to moving a member to the back ofthe actuator 9. For this purpose, the coils 42 of the actuator arephysically separated in two, so that they give room enough for the pin43, through the actuator 9. Such configuration makes it possible toposition a sensor 21 at the top of the rotor 13, since the outlet of thepin 43 is not required at the top of the rotor 13. Then, the utilizationof any type of sensors can be considered like those described in thepresent application, without being limitative.

FIG. 15 shows an alternative to the servo-actuator 1 of FIG. 12 forwhich the sensor 21 comprises a diametrically magnetized cylindricalfield emitter 50, integral with the pin 43, in the vicinity of which afield receiver 41 comprising two magnetism-sensitive members capable ofmeasuring the tangential and radial or tangential and axial componentsof the emitted magnetic field is positioned in a stationary way. Thefield receiver 51 then comprises a processing circuit capable of makingthe combination of the two components in order to determine the rotationangle of the pin 43. In such a configuration, the field receiver 51 ofthe sensor 21 being shifted with respect to the pin 43, the pin 43 canthus be protruding outside towards the top of the actuator 9 and canthus be fixed to a member to be moved.

FIG. 16 shows the servo-actuator 1 according to a fifth embodiment ofthe sensor 21 and without any overmolding or lid (that are hidden). Theyoke 15 of the actuator has a bipolar cylindrical magnet 52 with axialmagnetization. In the axis of this magnet 52, corresponding to theintersection of the middle plane of symmetry of the stator poles 11, isplaced a Hall effect probe 24, which has further at least two Hallsensitive elements. Such sensitive element sense the induction generatedby the magnet 52 according to two orthogonal direction. The outputsignal obtained is not disturbed by the cylindrical magnet 52 since thesensitive elements of the Hall probe 24 are placed close to the axis ofthe magnet 52, an area free of any magnetic induction coming from theactuator.

The probe is for instance the Melexis 90316 type, with an Integratedmagneto-concentrator that concentrates the applied magnetic flux densityparallel to the IC surface. At the boundaries of themagneto-concentrator structure, an orthogonal component proportional tothe applied flux density can be measured by two pairs of conventionalplanar Hall plates located orthogonally under the magneto-concentratorand for each of the two directions parallel with the IC surface (X andY). The first part of the sensor encodes a mechanical angle into twosinusoidal signals with 90° phase shift (sine and cosine). The twosignals (Vx & Vy) proportional to the magnetic field are amplified,sampled and converted in the digital domain. The digital representationsof the two signals are then used to calculate the angle through anArctangent function applied on the ratio Vy/Vx. The Arctangent functionis implemented by a look-up table.

The cylindrical magnet 52 is laid on the ferromagnetic yoke 15 so thatthe flux coming from the magnet 52 goes into the yoke 15, having highpermeability, before going into the probe 24, ensuring an enhancement onthe magnetic signal sense by the probe 24. The yoke 15 has then thefunction of enhancement of the induction for the magnet of the actuatorand the magnet of the sensor.

In sensitive applications requiring a resilient return system to providefor the return to the initial position of the member to be controlled,such locking/unlocking system 44 further makes it possible to divide theresilient return system in two, thus separating the mechanical torque tobe overcome with the actuator for moving the member to be moved againstthe spring. FIG. 13 showing such a servo-actuator 1 equipped with areturn system, shows a first resilient return member 46 in the form of atorsion spring located under the actuator and applying its return coupleto the pin 43 controlled by the actuator 9. A second resilient returnmember 47 in the form of a torsion spring is located at the level of theactuator 9 rotor 13 and applies its return torque directly to the rotor13. Thus, when the locking system 45 is locked, the actuator has toovercome only a second resilient return member 47 to position the memberto be controlled. When the locking system is unlocked, both torques ofboth resilient return members 46 and 47 add up to reposition the memberto be moved in its initial position. This makes it possible to undersizethe servo-actuator 1 and to use it in applications which it would not befit for, without this locking/unlocking system 44.

1. A single phase electromagnetic servo-actuator comprising: a rotaryactuator configured to move a mobile member along a limited travelincluding a 2N ferromagnetic pole stator structure, N being equal to 1or 2, and at least one excitation coil, the stator structure being madeof a material with high magnetic permeability and a rotor havingferromagnetic yoke and a thin magnetized portion of 2N pairs of axiallymagnetized poles, in alternate directions, and a rotor angular positionsensor, the thin magnetized portion being a separate element from theferromagnetic yoke, wherein the position sensor includes a magneticfield emitter integral with the ferromagnetic yoke and a magnetic filedreceiver for a magnetic field of the magnetic field emitter stationaryrelative to the stator structure, wherein the magnetic field receiver isclose to the magnetic field emitter and is located in a middle plane ofsymmetry of at least one of the ferromagnetic stator poles, wherein themagnetic field receiver includes a probe configured to measure two planecomponents and positioned on an axis formed by an intersection of themiddle planes of symmetry of the poles, and wherein the magnetic fieldemitter includes a ring magnet, the ring magnet having an axial bipolarmagnetization, the ring magnet laying on the ferromagnetic yoke so thatmagnetic flux of the ring magnet goes into the yoke.